NADH Oxidase Activity of Mitochondrial Apoptosis-inducing Factor

Apoptosis-inducing factor (AIF) is a mitochondrial flavoprotein, which translocates to the nucleus during apoptosis and causes chromatin condensation and large scale DNA fragmentation. Here we report the biochemical characterization of AIF's redox activity. Natural AIF purified from mitochondria and recombinant AIF purified from bacteria (AIFΔ1-120) exhibit NADH oxidase activity, whereas superoxide anion (O₂ ^-) is formed. AIFΔ1-120 is a monomer of 57 kDa containing 1 mol of noncovalently bound FAD/mol of protein. ApoAIFΔ1-120, which lacks FAD, has no NADH oxidase activity. However, native AIFΔ1-120, apoAIFΔ1-120, and the reconstituted (FAD-containing) holoAIFΔ1-120 protein exhibit a similar apoptosis-inducing potential when microinjected into the cytoplasm of intact cells. Inhibition of the redox function, by external addition of superoxide dismutase or covalent derivatization of FAD with diphenyleneiodonium, failed to affect the apoptogenic function of AIFΔ1-120 assessed on purified nuclei in a cell-free system. Conversely, blockade of the apoptogenic function of AIFΔ1-120 with the thiol reagent parachloromercuriphenylsulfonic acid did not affect its NADH oxidase activity. Altogether, these data indicate that AIF has a marked oxidoreductase activity which can be dissociated from its apoptosis-inducing function.